No need for kNeedsReattachStyleChange for designMode change.

LayoutNG used kNeedsReattachStyleChange instead of kSubtreeStyleChange
on Document when changing designMode to force a re-attach. However,
Document::UpdateStyle() does not have separate behavior for the two. The
LayoutNG code triggers layout tree changes on style recalc
appropriately.

We would like to get rid of kNeedsReattachStyleChange for a full
separation of style recalc and layout tree building.

TEST=editing/ with LayoutNG enabled

Change-Id: Iac04f705e56233de6771da224fc9e8edc9a487be
Reviewed-on: https://chromium-review.googlesource.com/c/1312476Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604570}

third_party/blink/renderer/core/dom/document.cc

@@ -5898,11 +5898,9 @@ void Document::setDesignMode(const String& value) {
   if (new_value == design_mode_)
     return;
   design_mode_ = new_value;
-  StyleChangeType type = RuntimeEnabledFeatures::LayoutNGEnabled()
-                             ? kNeedsReattachStyleChange
-                             : kSubtreeStyleChange;
-  SetNeedsStyleRecalc(type, StyleChangeReasonForTracing::Create(
-                                style_change_reason::kDesignMode));
+  SetNeedsStyleRecalc(
+      kSubtreeStyleChange,
+      StyleChangeReasonForTracing::Create(style_change_reason::kDesignMode));
 }
 
 Document* Document::ParentDocument() const {